1. Field of the Invention
This invention relates generally to the field of optical fiber communications. More specifically, the invention relates to the use of electrical crosspoints to implement the add/drop multiplexing (ADM) function in optical fiber communications systems using frequency-division multiplexing (FDM).
2. Description of the Related Art
As the result of continuous advances in technology, particularly in the areas of networking including the Internet, telecommunications, and application areas which rely on networking or telecommunications, there is an increasing demand for capacity for the transmission of digital data. For example, the transmission of digital data over a network""s trunk lines (such as the trunk lines for telephone companies or for the Internet), the transmission of images or video over the Internet, the distribution of software, the transfer of large amounts of data as might be required in transaction processing, or videoconferencing implemented over a public telephone network typically requires the high speed transmission of large amounts of digital data. Typical protocols which are intended to support such transmissions include the SONET and SDH protocols. As applications such as the ones mentioned above become more prevalent, the use of these and similar protocols and the corresponding demand for transmission capacity will only increase.
Optical fiber is a transmission medium which is well-suited to meet this increasing demand for transmission capacity. Optical fiber has an inherent bandwidth which is much greater than metal-based conductors, such as twisted pair or coaxial cable, and protocols such as the OC protocol have been developed for the transmission of digital data over optical fibers.
However, because of its large inherent bandwidth, an optical fiber is most efficiently used when multiple users share the fiber. Typically, a number of low-speed data streams, for example transmitted by different users, are combined into a single high-speed channel for transport across the fiber. Similarly, when the high-speed channel reaches the destination for one of the low-speed data streams contained in it, the low-speed data stream must be extracted from the rest of the high-speed channel. The low-speed data streams shall be referred to as tributaries. Once multiple tributaries are combined into a high-speed channel, the corresponding portion of the high-speed channel shall be referred to as a xe2x80x9clow-speed channelxe2x80x9d that occupies a xe2x80x9clow-speed slotxe2x80x9d within the high-speed channel. Thus, a high-speed channel contains a number of low-speed slots, each of which may be occupied by a low-speed channel. Furthermore, each low-speed channel corresponds to a tributary, or possibly a group of tributaries.
A typical optical network consists of nodes which transmit high-speed channels to each other over optical fibers. The tributaries may be fed to and received from these nodes via a number of communications channels, including branch fibers, metal conductors, or even wireless communications channels. In addition to transporting low-speed channels through the node (the xe2x80x9cpass-throughxe2x80x9d function), nodes typically also combine incoming tributaries to the high-speed channel (the xe2x80x9caddxe2x80x9d function) and extract outgoing tributaries from the high-speed channels (the xe2x80x9cdropxe2x80x9d function). These functions are commonly referred to as add-drop multiplexing (ADM).
Increasing the ADM functionality of the nodes in a network enhances both the applicability and the reliability of the network by increasing the number of applications, network configurations, and types of protection mechanisms that may be implemented by the network. For example, as described above, basic add, drop, and pass-through functionality supports the addition and extraction of tributaries to and from low-speed slots within high-speed channels. This enables a variety of network configurations, including point-to-point, linear chain, ring, and ring-to-ring configurations. More advanced ADM functionalities include drop-and-continue, in which a low-speed channel is both dropped as a tributary from one high-speed channel and continued on a low-speed slot of another high-speed channel; broadcast, in which a low-speed channel is dropped from a high-speed channel but then broadcast to multiple tributaries rather than just a single tributary; and multicast, in which a single tributary is added to multiple low-speed slots in one or more high-speed channels. These functionalities enable additional services, such as video and other Internet applications, to be deployed on top of the network configurations listed above. The added flexibility also facilitates the use of redundancy and the reconfiguration of the network with minimal disturbance to the on-going operation of the network.
However, the manner in which the ADM functionality is implemented in a particular network will depend in part on how the low-speed channels are combined to form a high-speed channel. Two widely used approaches to combining low-speed channels are wavelength division multiplexing (WDM) and time division multiplexing (TDM). In WDM, each low-speed channel is placed on an optical carrier of a different wavelength and the different wavelength carriers are combined to form the high-speed channel. Crosstalk between the low-speed channels is a major concernin WDM and, as a result, the wavelengths for the optical carriers must be spaced far enough apart (typically 50 GHz or more) so that the different low-speed channels are resolvable. In TDM, each low-speed channel is compressed into a certain time slot and the time slots are then combined on a time basis to form the high-speed channel. For example, in a certain period of time, the high-speed channel may be capable of transmitting 10 bits while each low-speed channel may only be capable of transmitting 1 bit. In this case, the first bit of the high-speed channel may be allocated to low-speed channel 1, the second bit to low-speed channel 2, and so on, thus forming a high-speed channel containing 10 low-speed channels. TDM requires precise synchronization of the different channels on a bit-by-bit basis (or byte-by-byte basis, in the case of SONET), and a memory buffer is typically also required to temporarily store data from the low-speed channels.
In the case of WDM, one approach is to implement the ADM functionality entirely in the optical domain. This avoids having to convert the high-speed channel from optical to electrical form, but has a number of other significant limitations. First, as described previously, the wavelengths for each of the optical carriers in a WDM system typically are spaced far apart (e.g. 50 GHz or more). As a result, the number of different optical carriers is limited and if each carrier corresponds to a tributary, as is typically the case, the total number of tributaries is also limited. Furthermore, if the bandwidth capacity of the fiber is to be used efficiently, each tributary must have a relatively high data rate due to the low number of tributaries, thus preventing add-drop at a fine granularity. For example, if the high-speed channel has a total capacity of 10 Gigabits per second (10 Gbps) and is allotted a bandwidth of 200 gHz, then current WDM systems will typically be limited to no more than four tributaries, each of which will be 2.5 Gbps in order to meet the overall bit rate of the high-speed channel. However, this means that the tributaries can only be added or dropped in blocks of 2.5 Gbps. Since many data streams occur at a much lower bit rate, such as at 155 Megabits per second (Mbps) for OC-3 tributaries, it is often desirable to add and drop at a granularity which is finer than what WDM can support.
The current state of technology also limits the practicality of all-optical ADM. In all-optical approaches, the channels typically are not regenerated as they pass through each node in the network and will continuously deteriorate until they reach their final destination. As a result, the entire network must be designed assuming deterioration along the worst-case path through the network. In contrast, if a channel is regenerated at each node, the network may be designed based only on node-to-node deterioration, regardless of the total number of nodes in the network. As another example, current technology makes it difficult to route a channel occupying a slot at one wavelength to a slot at a different wavelength. This severely limits the ADM functionality that may be implemented since each low-speed channel is not freely routable to any low-speed slot. For example, if a low-speed channel occupies a low-speed slot of a particular wavelength on an incoming high-speed channel, that low-speed channel can only be passed through to another high-speed channel if that channel""s low-speed slot at that particular wavelength is unoccupied, regardless of how many other low-speed slots at other wavelengths are available.
An alternate approach to implementing ADM functionality for WDM systems is based on converting the optical high-speed channels to electrical form and then performing the ADM function electrically. This approach, however, is expensive since it requires significant amounts of both optical and electrical devices. WDM is an inherently optical approach and requires optical devices to implement. On the other hand, an electrical ADM would require significant electrical devices to implement. Combining the two would require both sets of devices and would additionally require optical-to-electrical (O/E) and electrical-to-optical (E/O) converters, typically one set for each wavelength used in the WDM.
As a result of the disadvantages described above, ADM capabilities in current WDM systems are often fixed or limited. For example, add/drop connections between tributaries and high-speed channels may be fixed when a node is installed and may be changed only by a corresponding change in hardware. As another example, the add/drop functions may be implemented only for a subset of the tributaries connected to a node. Alternately, a node may be able to implement only a subset of all possible connections between tributaries and high-speed channels. These compromises reduce the overall ADM functionality of the node and its flexibility within a network.
Implementing ADM capabilities for TDM networks also has significant disadvantages. First, as mentioned above, the TDM approach is strongly time-based and requires precise synchronization of the channels entering and exiting the ADM to a common reference clock. As a result, TDM systems require significantly more complex timing recovery, leading to increased overall cost. In addition, since the tributaries typically are combined on a bit-by-bit (or byte-by-byte) basis, TDM systems are heavily dependent on the bit rates of the individual tributaries and have difficulty handling tributaries of different bit rates. As yet another disadvantage, TDM systems generally require significant amounts of buffer memory since bits from the tributaries typically must be temporarily stored before they can be properly sorted and time-synchronized to form a high-speed channel. These required buffers add to the cost of implementing an ADM within a TDM system.
Thus, there is a need for an inexpensive node that provides a broad range of ADM capabilities for optical communications networks, in particular including the functionalities of adding, dropping, drop-and-continue, and pass-through of a low-speed channel to/from a low-speed slot to/from any other low-speed slot. The node preferably implements the ADM functionalities independent of bit rate, format, and protocol of the various channels. Furthermore, the node should be able to handle large numbers of fine granularity tributaries and in a spectrally-efficient manner. There is further a need for a node which regenerates the channels passing through it.
In accordance with the present invention, an FDM node for use in optical communications networks includes an O/E converter, a frequency division demultiplexer, an E/O converter, a frequency division multiplexer, and an ADM crosspoint. In the high-speed receive direction, the O/E converts a first optical high-speed channel to a first electrical high-speed channel. The frequency division demultiplexer is coupled to the O/E converter and frequency division demultiplexes the first electrical high-speed channel into a first plurality of low-speed channels. In the transmit direction, the frequency division multiplexer receives a second plurality of low-speed channels and frequency division multiplexes them into a second electrical high-speed channel, which is then converted by the E/O converter to a second optical high-speed channel. The ADM crosspoint has a plurality of low-speed inputs and low-speed outputs which are respectively coupled to the frequency division demultiplexer and frequency division multiplexer. The ADM crosspoint additionally has a plurality of tributary inputs and tributary outputs. The ADM crosspoint switchably couples any low-speed input to any low-speed output, any low-speed input to any tributary output, and any tributary input to any low-speed output, thus implementing an add/drop multiplexing (ADM) function for the optical high-speed channels.
This approach is particularly advantageous because the use of frequency division multiplexing results in the efficient combination of low-speed channels into a high-speed channel and the efficient separation of a high-speed channel into its constituent low-speed channels. For example, since the multiplexing occurs in the electrical domain rather than the optical one, this approach requires only a single optical to electrical conversion (e.g., the optical high-speed channel into an electrical high-speed channel), whereas approaches like WDM would require multiple optical to electrical conversions (e.g., one for each wavelength), with a corresponding increase in the equipment required. Furthermore, since the multiplexing occurs in the frequency domain rather than the time domain, this approach does not have stringent synchronization requirements and does not require memory buffers as would be the case with TDM approaches.
The efficient conversion between optical high-speed channels and electrical low-speed channels enables the use of an electrical ADM crosspoint to implement the ADM functionality of the FDM node. This yields further advantages since a crosspoint is more flexible than other ADM solutions. In particular, a crosspoint can be configured to connect any input to any output. As a result, in addition to the basic add, drop, and pass-through functions, the ADM crosspoint can implement any combinations of the above, including multicasting. This flexibility allows a single FDM node to be configured in a variety of ways to support a variety of network configurations. It also allows the FDM node to be easily reconfigured while in service. This facilitates the implementation of system reconfigurations with minimal disturbance to in-service traffic and also facilitates the implementation of fault-tolerance by enabling data streams to be efficiently re-routed to redundant hardware in the case of failure of the primary hardware.
In another aspect of the invention, a method for adding a tributary to an optical high-speed channel includes the following steps. The tributary is received at one of the tributary inputs of the ADM crosspoint. The crosspoint is configured to couple the tributary input to one of its low-speed outputs to produce a low-speed channel from the tributary. The low-speed channel is frequency division multiplexed with a plurality of other low-speed channels to produce an electrical high-speed channel, which is then converted to an optical high-speed channel.
In another aspect of the invention, a method for dropping a tributary from an optical high-speed channel includes the following steps. The optical high-speed channel is received and converted to an electrical high-speed channel, which is frequency division de-multiplexed into a plurality of low-speed channels, one of which includes the tributary to be dropped. The low-speed channel of interest is received at one of the low-speed inputs of the ADM crosspoint, which is configured to couple that low-speed input to one of the tributary outputs, thus producing the tributary at the tributary output.
In yet another aspect of the invention, a method for passing a low-speed channel included in a first optical high-speed channel through to a second optical high-speed channel includes the following steps. The first optical high-speed channel is received and converted to a first electrical high-speed channel, which is frequency division de-multiplexed into a plurality of low-speed channels. One of the low-speed channels is to be passed-through to a second optical high-speed channel. The low-speed channel of interest is received at one of the low-speed inputs of the ADM crosspoint. The crosspoint is configured to couple that low-speed input to one of the low-speed outputs to produce a second low-speed channel at the low-speed output. The second low-speed channel is frequency division multiplexed with a plurality of other low-speed channels to produce a second electrical high-speed channel, which is then converted to optical form producing the second optical high-speed channel.